1. Field of the Invention:
This invention relates to an optical picture reader suitable for use, for example, in a facsimile or optical character recognition apparatus or an image processing apparatus for a copying machine or the like.
2. Description of the Prior Art:
An optical picture reader employed for example in an optical character recognition apparatus or the like comprises an optical system adapted to illuminate a picture to be read and to form an optical image corresponding to the thus-read picture and photo detectors for receiving the optical image formed by the optical system.
Such an optical system may for example be constructed of a tubular light-source lamp, a gutter-like reflector surrounding the lamp, and one or more optional plane reflectors interposed in the optical path extending to photo detectors as needed. By the light-source lamp and gutter-like reflector, a long, narrow, strip-like illuminated zone which extends in a widthwise direction is formed on a picture to be read. The picture to be read is shifted relative to the illuminated zone, thereby scanning the picture at the illuminated zone and forming an optical image of the whole picture.
As pictures to be read, there are those having various colors such as black color, blue color, red color, orange color and green color at their picture areas. Hence, it is generally desirable that reading can be always effected with high accuracy irrespective of the kind of the color of a picture to be read.
An optical system constructed, for example, by using a halogen incandescent lamp as a light-source lamp and an aluminum reflector has such characteristics that the spectral sensitivity increases almost linearly in the visible region as indicated by Curve I in FIG. 1, which diagrammatically illustrates the spectral sensitivity characteristics of a light source composed of a halogen incandescent lamp and photo detectors in comparison with spectral luminous efficiency. In order to improve the reading accuracy by using such an optical system, it is desirable that the combined spectral sensitivity characteristics of the optical system and photo detectors closely resemble the spectral luminous efficiency of a man for photopic vision, which is indicated by Curve II in the same figure. For this purpose, it is desirable that the spectral sensitivity of the photo detectors has such characteristics as shown by Curve III in the same figure.
As photo detectors on the other hand, photo detectors making use of the photovoltaic effect of a p-n junction of silicon such as silicon photo diodes and silicon photo transistors are used widely. These photo detectors have sensitivity, generally, in a wavelength region of 400-1100 nm with the maximum peak wavelength falling within a range of 800-900 nm. Their spectral sensitivity hence contains an oversensitive part, i.e., a part, which is far more sensitive compared with the spectral luminous efficiency of a man, in a longer wavelength range of the visible range. When photo detectors having such spectral sensitivity characteristics are used to read a picture having a red color by way of example, their sensitivity to light reflected from an area of the red color in the picture is so high that the photo detectors show substantially the same characteristics as their characteristics to light reflected from an area of a white color in the picture. This results in a problem of lowered reading accuracy.
As a method for overcoming the above-described problem, it has already been known to interpose a suitable filter in the optical path extending from the light-source lamp to the photo detectors so as to modify the spectral characteristics of the resulting optical image, whereby the quantity of light, which has a wavelength corresponding to the oversensitive range and enters the photosensitive detectors, is reduced to improve the reading accuracy.
The above method have however developed other problems. Since the filter is susceptible to dust deposition on its surface and also to scratches, special care must be exercised upon handling the filter. The filter requires a special bracket for holding same, whereby the assembly of the apparatus is rendered more complex. Moreover, a filter of this sort generally has a multi-layered coating. It is hence expensive, thereby increasing the manufacturing cost of the apparatus, and has poor heat resistance and thus a shorter service life.
There have recently been developed photo detectors in each of which an infrared absorbing filter is provided with a planar silicon sensor so as to make its spectral sensitivity closer to the spectral luminous efficiency of a man. However, there are still oversensitive parts in both shorter and longer wavelength ranges in the visible region as shown by Curve IV in FIG. 1 even in the case of such photo detectors. Their sensitivity is still very high, for example, to blue colors corresponding to 400-500 .mu.m or to red colors corresponding to 600-700 .mu.m so that they involve a problem of low reading accuracy.
CCD (charge coupled device) line sensors, which have found wide-spread utility as photo detectors having elongated light-receiving surfaces, also have oversensitive parts in both shorter and longer wavelength ranges in the visible region as indicated by Curve V in FIG. 1. They are hence accompanied by a similar problem.
On the other hand, a copying machine such as electrophotographic copying machine generally includes an optical system for illuminating an original document to form an optical image corresponding to the original document, a photosensitive member adapted to be exposed to the optical image of the original document so as to form an electrostatic latent image thereon, a developing means for visualizing the electrostatic latent image with a toner, and a fixing means for transferring a toner image formed by the above development onto a transfer material such as a sheet of paper and then forming a fixed image there.
The above-described optical system may be composed, for example, by a tubular lamp for illuminating an original document, a gutter-like mirror surrounding the lamp, one or more optional plane mirrors interposed in the optical path extending to a photosensitive member as needed, and a image-forming lens for focusing an optical image of the original document on the photosensitive member. By the original document illuminating lamp and gutter-like mirror, there is formed an elongated strip-like illumination zone which extends in the widthwise direction of an original document table made of transparent glass. The original document or optical system is shifted so that the illumination zone is scanned relative to the original document to form an optical image of the entire original document.
As original documents to be copied, there are those having various colors such as black color and red color at their marked areas. Hence, it is generally desirable that their copies formed by a copying machine are of good quality having the same density irrespective of the types of the colors in the marked areas of the original documents.
However, conventional copying machines are accompanied by a problem that the density of a marked area of a resulting copy corresponding to a marked area of a red color in an original document becomes considerably lower compared with the density of another marked area of the same copy corresponding to another marked area of a black color in the original document. Organic photosensitive members (may hereinafter be called merely "OPC photosensitive members") which comprise organic photoconductor or selenium-base photosensitive members made of a photoconductor of selenium-arsenic alloy are widely used, for example, in electrophotographic copying machines. Of these, OPC photosensitive members made, for example, of a eutectic mixture such as a triphenylmethane mixture and a thiapyrylium salt and selenium-base photosensitive members made of AsSe have oversensitive parts, i.e., parts, which are far more sensitive compared with the spectral luminous efficiency, in a wavelength region of 550-650 nm, namely, a wavelength region close to red colors. When a copying machine making use of a photosensitive member having such spectral sensitivity characteristics is used to form a duplicate copy of an original document having a marked area of a red color, its sensitivity to light reflected from the area of the red color in the original document is so high that it exhibits characteristics similar to its characteristics to light reflected from an area of a white color in the original document. As a result, the density of a copied area corresponding to the marked area of the red color in the original document is low in the duplicate copy, thereby developing a problem that its sharpness is lower than that of copied areas corresponding to marked areas of a black color in the same original document. On the other hand, the spectral sensitivity of OPC photosensitive members made, for example, of polyvinylcarbazole and trinitrofluorenone and seleniumbase photosensitive members made, for instance, of As.sub.2 Se.sub.3 or SeTe contain oversensitive parts, i.e., parts, which are far more sensitive compared with the spectral luminous efficiency, in a wavelength region of 400-550 nm. When a copying machine making use of a photosensitive member having such spectral sensitivity characteristics is used to form a duplicate copy of an original document written in a blue ink or an original document prepared as a so-called blue print for example, its sensitivity to light reflected from a marked area of a blue color in the original document is so high that a problem similar to that described above is developed.
As a method for overcoming the above-described problem, it has already been known in a conventional electrophotographic copying machine to interpose a suitable filter in the optical path extending from the original document illuminating lamp to the photosensitive member so as to modify the spectral characteristics of the optical image of the original document, whereby the quantity of light reflected from a marked area of a red or blue color in the original document is reduced on the photosensitive detectors so as to compensate the insufficient density of the marked area of the duplicate copy corresponding to the marked area of the red or blue color in the original document.
The above method have however developed other problems. Since the filter is susceptible to dust deposition on its surface and also to scratches, special care must be exercised upon handling the filter. The filter requires a special bracket for holding same, whereby the assembly of the apparatus is rendered more complex. Moreover, a filter of this sort generally has a multi-layered coating. It is hence expensive, thereby increasing the manufacturing cost of the apparatus, and has poor heat resistance and thus a shorter service life.